Many photographic elements include silver halide emulsions having one or more bis-benzothiazolocyanine dyes, in combination with conventional supersensitizers, to spectrally sensitize cyan emulsions to red light. These conventional supersensitizers were effective, but their stain propensity was high. In many emulsions they are retained in the processed and developed emulsion, resulting in an undesirable pink-colored stain in the final image. There is a need for dyes which are more readily removed from the film, while at the same time maintaining good efficiency and proper spectral sensitivity.
The ability of certain cyanine dye combinations to enhance red sensitivity in a superadditive manner ("supersensitization") is well known. Many emulsions incorporate a cyanine dye that is highly insoluble in water and many common organic solvents, requiring an undesirable organic solvent (2-phenoxyethanol) as a vehicle. Little if any of this dye is removed from the emulsion during the development process. An alternative supersensitizer with improved water solubility would be of great practical value, both from the standpoint of solution preparation and post-process retention.
Tabular grain emulsions generally require 2 to 4 times more dye per mass of coated silver than conventional emulsions of similar speed. This places a much greater premium on efficient dye removal. For optimum performance, both the principal dye and its supersensitizer should be designed for easy removal.